A vehicle has a battery mounted thereon as a power source for its electric devices, and is provided with a power supply circuit for providing a predetermined constant voltage to electric devices, regardless of the state of the battery. Meanwhile, electromagnetically induced noises and surges generated from an actuator, relay and solenoid controlled by an ignition device or an electronic control device are often superposed on a power source line of the battery which provides power to electronically controlled devices. It is therefore important to remove and reduce these noises and surges generated in the power source line of the battery to prevent the malfunction and damage of the electronically control devices, and improve their reliability.
To this end, a constitution in which a surge-preventing circuit, a switching circuit, and a constant voltage circuit (smoothing circuit) are sequentially connected to the battery power source is employed, and is disclosed in, for example, patent document 1. In these power supply circuits, a constitution in which the surge-preventing circuit functions to turn off the switching circuit when a surge is applied to keep the constant voltage circuit from operating has been used.